Patent document 1: JP 2007-237768 A
Vehicles each have a plurality of ECUs (Electronic Control Units). The ECUs need a power supply in a standby state of ACC-OFF, possibly causing a battery of the vehicle to run out. For example, an in-vehicle navigation ECU has generally a DRAM (Dynamic Random Access Memory) as a main memory; data stored in the DRAM may disappear when the battery of the vehicle runs out. Patent document 1 proposes a countermeasure to monitor whether the battery of the vehicle is going to run out. That is, when the battery is going to run out, the power source is shut down after evacuating or saving the data from the main memory of the ECU to a nonvolatile memory.
However, such a countermeasure requires the data saved in the nonvolatile memory to return to the main memory when subsequently being re-activated by switching from the standby state of ACC-OFF into the usual state of ACC-ON. This unfortunately increases an activation time. Thus, the ECU is provided with a backup power source receiving continuous power supply from the battery of the vehicle; the backup power source maintains a power supply to the main memory. This permits the high-speed start of the navigation ECU when the usual state of ACC-ON replaces the standby state of ACC-OFF. Such a configuration, however, maintains unchanged the output voltage from the backup power source to the main memory even when the standby state replaces the usual state. This needs significant dark current during the standby state, possibly causing the battery of the vehicle to run out.